Vibration systems, commonly known as shaker tables, are used for testing the vibration tolerance of manufactured products which are subjected to vibration in their operating environments or in shipping. For example, products used in vehicles and machinery are often subjected to high levels of vibration and require 10 testing to ensure their reliability under those conditions. Shaker tables are often used in the design, development and manufacturing of a product. They are used by development and testing laboratories and on assembly lines. It is also common for shaker tables to be mounted within, or protrude into, a thermal chamber to combine vibration with temperature cycling in the testing process. Since the testing of a product often includes the vibrating of components of a product, as well as the product itself, the generic object of a vibration test is hereinafter referred to as a specimen.
Some types of shaker tables, typically those providing vibrations in six axes, include a platform or mounting table upon which a specimen, or a fixture for attaching a specimen, is mounted. The table is usually mounted on flexible supports which permit it to vibrate freely and independently of the environment. Actuators, also known as exciters or vibrators, are attached to the table to produce vibrations. The table couples the vibrations from the actuators to the specimen.
Typically, in these types of tables, the actqator is an impact device which produces very high forces of very short duration. Very short duration, very high magnitude pulses in the time domain translate into broadband spectra in the frequency domain (as described mathematically by the Fourier Transform). The physical properties of the table cause it to respond to the different frequencies in the impact spectrum in different ways. In other words, the physical properties of the table can be ahosen to resonate with, or enhance the amplitude of, certain frequencies and to suppress others. The properties can cause the table to vibrate or ring (like a bell) at a particular frequency for a relatively long time, or to damp (or quench) the vibration in relatively short time. Also, depending on the location and orientation of the actuators attached to the table, as well as the dimensions and properties of the table, the table may have different modes of vibration. As used herein, a mode of vibration means a particular frequency, wavelength and orientation of vibration. For example, the center of a bar supported at its ends, when struck with a sharp impact, may vibrate at a particular frequency while the ends remain stationary. The bar may also be able to vibrate at another frequency such that a point near the center of the bar also remains stationary while the bar vibrates between the center and the ends. At other frequencies, there may be multiple stationary points along the length of the bar with vibrations occurring between each of the stationary points. Each of these conditions is referred to as a mode of vibration. The stationary points are called nodes and the points of maximum amplitude of vibration are called anti-nodes. The preceding describes multiple modes of vibration in one dimension. However, a shaker table has at least three dimensions if it comprises a single plate and may have more if it is a laminated structure having multiple layers of different materials. Hence, a shaker table may have many modes of vibration in each of multiple dimensions. Since an impact produces a broad range of frequencies, it can excite many modes of vibration in a shaker table. The properties of the table can be selected to enhance or suppress particular modes of vibration and thereby tune the behavior of the table.
Shaker tables have been designed and built with many various combinations of dimensions, layers and materials for the purpose of producing and tuning multimodal vibrating characteristics to meet the testing requirements of specimens ranging from semiconductor devices to spacecraft. Tables have ranged from very stiff structures with very little damping to very flexible structures which are heavily damped.
U.S. Pat. No. 3,369,393 (1968), to E. W. Farmer, describes a light weight, stiff test fixture comprising upper and lower horizontal plates separated by perpendicular vertical walls. The walls are arranged on a horizontal grid pattern to form cells which may have different horizontal dimensions. The lower plate is attached to an actuator for providing shaking motions. The upper plate provides a mounting surface for specimens. The fixture provides a dynamically stiff coupling between the upper and lower plates over a frequency range of interest.
U.S. Pat. No. 3,686,927 (1972), to Terry D. Sharton, discloses light weight, flexible test fixtures having various dynamic behaviors. The test fixtures comprise first and second members coupled by a system of flexible members. The first member is attached to an actuator fir providing shaking motions. The second member provides for mounting the specimens. The flexible members are chosen to provide a desired dynamic coupling fun ion between the first and second members. The flexible members described include systems of wires, beams, plates and trusses.
U.S. Pat. Nos. 4,181,027, 4,181,028 and 4,181,029 (1980) to C. F. Talbott, Jr., describe vibrating systems comprising a driving structure, a visco-elaptic structure coupled to the driving structure, and a driven structure coupled to the visco-elastic structure. The driving structure is caused to vibrate by pneumatic actuators. The specimens are mounted on the driven structure. The visco-elastic structure couples the vibration of the driving structure to the driven structure and in turn to the specimens. These patents describe various ways of clamping the driving and driven structures together, including springs, bolts and pneumatic devices. various ways of altering the stiffness and damping characteristics of the visco-elastic material are also described, including heating and cooling of the material.
U.S. Pat. No. 4,735,089 to Richard L. Baker, et al., (1988) describes a shaker table comprising a table base, a plurality of damping layers mounted on the table base, and a table top mounted on the damping layers. Vibrating assemblies apply vibrating motions to the table base. The specimens are mounted on the table top. The damping layers provide dynamic coupling between the table base and the table top. The damping layers comprise laminated panels of honeycomb bonded together with a flexible adhesive. The table top is segmented into a plurality of sections.
U.S. Pat. No. 5,412,991 (1995) to Gregg K. Hobbs describes a stiff shaker table comprising a thick stiff plate having multiple internal weight-reducing cavities. Specimens are mounted on the plate. Vibrating actuators are attached tb the plate to provide vibrating motion. An embodiment using a closed-cell honeycomb panel in place of the plate is also described.
The previously described inventions disclose shaker table designs with structures ranging from stiff to flexible and with various levels of damping. Some also describe ways of modifying the table design to change the performance characteristics of the table. Others describe control systems for dynamically varying actuator operation or for varying the preloading or temperature of the visco-elastic material. In each of these inventions, the table structures are practically fixed by the design and are not easily changed. However, it is often necessary or desirable to test specimens of substantially different sizes, weights and dynamic behaviors under widely varying test conditions. Consequently it is desirable to have a vibration system which can be quickly and easily modified to adapt to the testing requirements of different specimens.
It is therefore an object of the present invention to provide a modular vibration system which can be easily adapted to testing any of a large variety of specimens under a wide range of conditions by the use of interchangeable system components or modules.